In-Process Measurement Of Fast Aspherics

AbstractIn the fabrication of fast aspheric mirrors having diameters in the 20-30 inch range, it ceases to be practical to introduce the required correction by polishing alone. One is faced, therefore, with aspheric figure control of a surface which cannot be tested optically. To overcome this problem we have adopted the use of precision profile monitors which can provide a prepolishing surface having an accuracy of better than one micron. Upon reaching the polished stage we have found that a wire tester provides the most convenient means for figure control. However, for the final stages of polishing, where local hand correction may be required, it is desirable that the wire tester be replaced by a full aperture interferometric technique; usually a null lens. Described in this paper is three stage monitoring procedure which uses a profile monitor, a wire tester and a null lens. Accuracy convenience of each technique are discussed. Also, suggestions are made concerning the optimum point at which one instrument should be exchanged for another.IntroductionIn the fabrication of severe aspheric mirrors the surfaces of which depart from the starting spherical configuration by an amount which is not practical to introduce by a polishing process alone, one must utilize a monitoring device which does not require a reflecting surface for analysis. Additionally, it is desirable that the surface in process be brought sufficiently close to prescription so that subsequent interpretation of interferometric data is obvious and unambiguous. The requirements imposed upon the DOF* with the fabrication of a 27 inch diameter F 1.4 ellipsoidal mirror, surface which departed from the initial edge tangent spherical surface by approximately 400 microns, provides a good illustration of this requirement. In performing this task the DOF* has followed a three phase diagnostic program designed to monitor the progress and accuracy of the mirror throughout fabrication process. This program utilized a Surface Profile Monitor while generating the basic aspheric contour with a free abrasive grinding process, a Wire Test Apparatus once polishing operations were commenced and ultimately a Null Lens/Interferometer for final quantitative analysis. While all of these instruments have been available, in one form or another, for many years their use both with modern metrology equipment and in conjunction one another is worthy of note.Surface Profile MonitorThe Surface Profile Monitor is a mechanical instrument capable of discerning small departures from a specific, pre-determined sphere. This device, as designed and built at the DOF, employs a ball pivot precisely positioned at the mean center of curvature; in this case a concave F 1.4 ellipsoidal mirror of 27 inch diameter. Suspended from the ball pivot is a pendulum to which an appropriate interrogating sensor is attached. The accuracy of the instrument determined by:1. The degree of precision achieved when locating the pivot point at mean center of curvature of the mirror under test.2. The effects of moments created when the interrogating pendulum is swung outward from the gravity vector to the 10° angle required complete a full radial scan of mirror's surface.3. The resolution and reliability of the interrogating sensor itself.4. The variables resulting from thermally induced distortions of the instrument structure while subjected to the lab environment.Addressing the above considerations regarding precise location of the ball pivot led ultimately to instrument configuration depicted in Figure I. In this design the ball pivot is rigidly held atop a thin- walled slotted aluminum cylinder; the base of which comprises an integral centering fixture and support pedestal. The centering fixture consists simply of a precisely machined lucite disc which fits into the central perforation of this specific mirror; thus providing repeatable radial registration of the instrument with respect to the mirror's optical axis. Positive registration along this axis is provided by the support pedestal which rests upon the fine ground surface of mirror itself.^Development Optics Facility of the Air Force Weapons Laboratory operated by International Laser Systems, Inc. 70 I SPIE Vol. 171 Optical Components: Manufacture 8- Evaluation (1979)Downloaded From: http://proceedings.spiedigitallibrary.org/ on 12/13/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx